Axions “are excellent dark matter candidates,” said Asimina Arvanitaki, a theoretical physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada.

Axions would clump together in exactly the ways we expect dark matter to, and they have just the right properties to explain why they’re so hard to find — namely, they’re extremely light and reluctant to interact with regular matter.

Earlier this year, a group of scientists reported that they might have spotted evidence of axions being produced by neutron stars — collapsed stars that are so dense, a tiny sample little bigger than a grain of sand would weigh as much as an aircraft carrier.

Ever since the 1980s, physicists have thought that if axions do exist, they should be produced inside the hot cores of neutron stars, where neutrons and protons smash together at high energies.

(This property forms the basis for earthbound axion searches such as the Axion Dark Matter Experiment, which uses powerful magnets to try and spot the transformation in action.) Axions flying through the neutron star’s magnetic field would be transformed into X-ray photons.

Most known neutron stars are rapidly spinning pulsars, which release copious amounts of X-rays anyway — no axions needed.

In the study, published in Physical Review Letters, Safdi and his colleagues suggest that all but one of these neutron stars show an excess of higher-energy X-rays that “could possibly be explained by the existence of axions,” Safdi said.

Safdi’s team plans to investigate the matter further with additional instruments, like NASA’s NuSTAR X-ray telescope, which can observe higher-energy X-rays than other space telescopes can see.

The NuSTAR telescope is sensitive to the high-energy X-rays that would provide stronger evidence for the existence of axions.

(A definitive link cannot yet be drawn, however.) Elsewhere, black holes have been touted as prime laboratories for probing the existence of axions by looking for signs of a process called superradiance, a phenomenon where lightweight particles — such as axions — could slow the spin of a black hole anywhere from 10% to 90% by causing it to lose energy and angular momentum.

As axions have slowly become one of the most tantalizing dark matter candidates, researchers have come up with ever more elaborate ways to find a wisp of a particle that may not even exist

“Because axions or other dark matter-like particles are so feebly interacting, you need a big number somewhere to ratchet it up to something you could see,” said Thaler